Recently, as the social interest in the health care has increased, the development of gas sensors that are based on a metal oxide semiconductor and intended to detect harmful environmental gases have been actively carried out for the detection of volatile organic compound gases in the exhaled breath and the measurement of air quality. Such gas a sensor based on a metal oxide semiconductor senses a gas by measuring the electrical resistance which changes in the adsorption and desorption processes of a specific gas to be detected by the interaction with the oxygen ion adsorbed onto the metal oxide surface. In particular, a metal oxide gas sensor has an advantage of being easily miniaturized, and thus a study to mount the gas sensor to a portable device or a wearable device has recently been attempted from a commercial point of view. In addition, it also has an advantage that the price is inexpensive, and thus it is widely applied in society as a harmful environment gas detector, a breathalyzer, an air pollution detector, an anti-terrorism gas sensor, and the like. Particularly, in recent years, a possibility has been proposed that a variety of diseases such diabetes, nephritis, asthma, halitosis, lung cancer can be diagnosed by detecting a significantly small amount of a volatile organic compound gas, such as acetone, ammonia, nitrogen monoxide, hydrogen sulfide, or toluene, that is contained in the exhaled breath and associated with the biological metabolism using the superior detection capability of the metal oxide sensor. In practice, however, it is required to be able to sense such a biomarker gas having a significantly low concentration in a range of from 10 ppb (parts per billion) to 10 ppm (parts per million) with a high speed of a few seconds and a high sensitivity in order to diagnose the disease at an early stage by using the biomarker gas. In particular, it is required to react with a specific target biomarker gas among the thousands of mixed gases contained in the exhaled breath with a high sensitivity, and thus it is significantly important to develop a sensing material exhibiting high selectivity to the gas to be measured.
In order to equip a gas sensor based on a metal oxide semiconductor with ultrahigh sensitivity/high selectivity, the development of the gas sensor based on a variety of nanostructures including nanoparticles, nanofibers, and nanotubes have recently been studied. As mentioned above, the metal oxide-based gas sensor utilizes the surface reaction of the sensing material with the gas to be detected, and thus a higher sensitivity is expected as the surface area of the sensing material on which the reaction with the gas molecule to be detected takes place is wider. From this point of view, the nanostructure sensing material exhibits excellent gas detection characteristics since it has a relative wider area for the reaction with a gas as compared with a thick film material or a thin film material, and the nanostructure sensing material has a porous structure through which the gas molecules can sufficiently rapidly diffuse into the sensing material and thus high-speed response characteristics can be induced. In particular, in the case of a one-dimensional porous metal oxide nanotube having mesopores and macropores, the surface area can be expected to be from 2 to 10 times nanofibers having a thin film structure, thus high detection characteristics are expected, and pores having various sizes are distributed on the tube surface, thus the gas molecules freely moves as compared with the dense nanofiber and nanotube structures and the characteristics of the sensor can be maximized. Additionally, the catalytic effect can be maximized even with a small amount of catalyst if the catalytic nanoparticles are uniformly loaded on the one-dimensional porous nanotube without aggregation with one another. In addition, in order to maximize the catalytic effect, rather than a structure in which the catalyst is embedded in a dense sensing material so as not to be able to react with the gas, it is ideal that the sensing material is functionalized such that the catalyst is exposed to the surface thereof and thus the catalytic reaction with the gas is maximized. Such catalysts are largely classified into two types, and there are a metal catalyst such as platinum (Pt) or gold (Au) used in a chemical sensitization method in which the characteristics of the gas sensor are enhanced by increasing the concentration of the gas participating in the surface reaction by the use such a metal catalyst and a metal catalyst such as palladium (Pd), nickel (Ni), cobalt (Co), or silver (Ag) used in an electronic sensitization method in which the sensitivity is improved by a change in oxidation state due to the formation of a metal oxide such as PdO, NiO, Co2O3, or Ag2O.
As described above, although studies to utilize sensing materials formed by loading various nanoparticle catalysts together with the development of various nanostructures have been continued, it is the reality that a sensing material that is based on a oxide material semiconductor and can precisely sense a trace amount, less than hundreds ppb, of gas at a high speed has not yet been commercialized, and it is significantly important to develop a sensing material which can sense a trace amount of gas and to clearly recognize the pattern of the detected result by imparting selectivity to various kinds of gases for the realization of a exhaled breath sensor to diagnose the disease at the early stage.
From the viewpoint of the synthesis of a sensing material having a nanostructure, a number of studies on the method to manufacture nanostructures through a chemical vapor deposition method, a physical deposition method, and a chemical growth method have been carried out. However, these methods include a complicated and cumbersome manufacturing process upon the synthesis of nanostructures, and thus there are problems such as difficulties in mass production, an expensive process cost, and a long processing time, which are a major challenge to commercialization.
In addition, from the viewpoint of the nanoparticle catalyst to be loaded to the sensing material, the most effective catalytic action is induced when the catalyst is uniformly dispersed without aggregation in the entire area of the sensing material. In this respect, it is difficult to optimize the sensing characteristics since the aggregation of nanoparticles is hardly avoided during the synthesis of nanoparticles utilizing the polyol process and the loading by the mixing of the catalyst particles and the sensing material which are widely used in the conventional sensor field.
In order to overcome these disadvantages in the conventional sensor synthesis, an ideal nanostructure that is formed by a simple and effective method, has a wide surface area, and includes both mesopores and macropores which lead rapid diffusion and reaction of the gas and a process technology which can functionalize the sensor with a nanoparticle catalyst having a nano-size by thoroughly dispersing without aggregation are required. In addition, a process technology which satisfies the two matters described above at the same time and thus contribute to the development of a sensor which can selectively sense a significantly amount of biomarker gas contained in the actual human exhaled breath, recognize the pattern, and ultimately distinguish the patient with the disease.